The present disclosure is generally directed to toner processes and, more specifically, to preparation of toner compositions having phosphorescent components which may be useful for document security.
Fluorescent inks and dyes may be used as an authenticating feature in the document security industry. Secure documents, for example documents that are difficult to forge, may be conventionally created using inks that include fluorescent agents either alone or in combination with ordinary inks and/or pigments. Features printed using fluorescent inks are usually invisible under visible light, due to the colorless nature of the security inks or due to masking by other colorants in the document. Under ultraviolet illumination, however, the fluorescent features of the document are revealed in the form of a bright emission by the fluorescent dyes in the visible spectrum. For example, certain bank notes utilize visible features, such as holographic patches, microprinting and microtextures to conceal additional fluorescent threads and/or multi-colored emblems embedded in the bank note, which are only revealed under specific light frequencies. These features provide an increased level of security against counterfeiters by making the copying process of such a document more difficult.
A phosphorescent image may similarly be useful in electrophotographic applications, including for security and special effects. Phosphorescence is a type of photoluminescence related to fluorescence, but unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. Such a phosphorescent image would continue to emit light after external light sources were removed, which is not possible with fluorescent materials.
While commercial phosphorescent pigments exist, they are too large to be incorporated into toner particles, as median pigment sizes range from 5 to >50 microns, similar in size or larger than the toner. This fundamental limitation is due to a key physical principle: large particle size pigments are needed to maintain the phosphorescent material properties. Both chemical and conventional toner processes currently available will fail to incorporate these large pigments. Thus, it is currently not possible to incorporate such large pigment particles in an emulsion aggregation (EA) toner process.
Also, while it is possible to melt-mix pigment particles with a toner resin, due to the large size of the phosphorescent pigment, even if the toner were 20 or 30 microns in size, the pigment particles would make up the bulk of the toner. For example, a 35 micron toner with one 20 micron pigment particle would have a pigment loading of about 40%. Thus, it would be extremely difficult to jet such large toner particles having such a high pigment loading. Also, with such a large pigment, even a 20 to 30 micron toner would only have a few pigment particles in each toner particle. Statistically, the toner would be very inhomogeneous; many particles would have no pigment particle in them, while others would have one or perhaps two or three pigment particles. Thus, to date it has not been possible to directly prepare phosphorescent electrophotographic prints.
Methods for producing phosphorescent toners which are suitable for use in creating electrophotographic prints thus remain desirable.